This invention relates to a lid for a can-like container and a method of manufacturing such a lid and more specifically to lids for various can-like containers such as cans for holding beverage coffee, soup, food oil, seasoning, motor oil and other canned goods, the lids being made predominantly of a synthetic resin instead of metal and being of such a construction that the lid is openable without any tool such as can opener.
Applicant of the present application has earlier proposed a method of manufacturing a lid of the above-mentioned type which is made predominantly of a synthetic resin and is openable without any tool such as a can opener (Japanese Patent Application No. 11238/87).
This manufacturing method will now be described with reference to the drawings. As shown in FIG. 5, a multi-layer base 23 having gas barrier properties is inserted in one injection mold 19 (i.e., right-hand mold in this Figure) in such a manner as to cover a surface of this mold.
For example, as shown in FIG. 10, the multi-layer base 23 of the gas barrier type comprises at least three layers, that is, an inner base layer 39 of the gas barrier type made, for example, of an aluminum foil, and opposite outer surface layers 40 and 41 of a thermoplastic resin. As shown in FIG. 5, preferably, the multi-layer base 23 is preformed into a container-like shape having a bottom, an upstanding portion (peripheral wall) extending from the outer periphery of the bottom, and a flange extending from the upstanding portion in parallel relation to the bottom. The multi-layer base 23 is inserted in the above mold in a manner mentioned above.
As shown in the drawings, a thermoplastic resin layer 26, which serves to reinforce at least the upstanding portion and flange of the container-like multi-layer base 23 (hereinafter referred to as "preformed base 23") for the lid for a can-like container, is formed on the preformed base 23 in such a manner that the thermoplastic resin layer 26 extends from the outer peripheral edge portion of the bottom of the preformed base 23 to the end of the flange and further projects therefrom in parallel relation to the upstanding portion in a direction opposite to the upstanding direction of the upstanding portion (the layer 26 is hereinafter referred to as "a flap portion 27" including the above oppositely-projecting portion 26').
Three molds 12, 13 and 14 are formed on a core-side surface of the other injection mold 9.
FIG. 4A shows the positional relation between these three molds as viewed from the core side. As shown in this Figure, the mold 12 in the form of a circular strip is formed on the core-side surface of the injection mold 9 at its outermost portion, and the mold 13 in the form of a horseshoe-shaped strip is disposed inside the mold 12 and is spaced from the upper portion of the mold 12. The mold 14 similar in shape to the mold 13 is disposed inside the mold 13 in spaced relation thereto, the mold 14 having the same height as the mold 13 and having upper ends directed inwardly toward each other.
FIG. 4B is a top plan view of a horseshoe-shaped tab (finger grip) 15 made of a synthetic resin, and FIG. 4C is an enlarged cross-sectional view taken along the line A--A of FIG. 4B, and FIG. 4D is a side-elevational view of the tab 15. As shown in FIG. 4A, the tab 15 is beforehand mounted on the core-side surface of the injection mold 9, on which the molds 12, 13 and 14 are formed, in such a manner that an upper end of a fixing portion 16 of the tab 15 is held in abutting engagement with the inner surface of the circular mold 12, with a body 17 of the tab 15 interposed between the molds 13 and 14.
The front end portion (fixing portion) 16 of the tab 15 has an oval hole 18 extending therethrough, as shown in FIG. 4A.
As shown in FIG. 5, the tab 15 is mounted on the injection mold 9 in such a manner that the through hole 18 is disposed in registry with an injection port 11.
Since the fixing portion 16 of the tab 15 is stepped with respect to the tab body 17 as shown in FIG. 4C, there is a spacing between the rear or lower surface of the fixing portion 16 and the upper surface of the preformed base 23.
As described above, the preformed base 23 with the flap portion 27 (hereinafter referred to as "lid body 20") is set on the surface of the one (i.e, right-hand) injection mold 19 disposed on its cavity side (FIG. 5), and then a mold closing or clamping is carried out as shown in FIG. 6.
After this mold closing operation, an injection molding material 22 (which is hereinafter often referred to merely as "injection resin") is injected into a cavity 21 through a resin flow path 10 and the injection port (gate) 11 in the injection mold 9, as shown in FIG. 7.
Since the through hole 18 of the tab 15 is disposed in registry with the injection port 11 as described above, the molten injection resin 22 flows through the through hole 18 and further flows to the backside of the fixing portion 16 of the tab 15.
The injection resin 22 thus fed to the backside of the fixing portion 16 flows into an area delimited by the innermost mold 14 and also flows from the fixing portion 16 of the tab 15 in right and left directions (FIG. 4A) into an area formed between the circular mold 12 and the mold 13. The manner of such flow is shown in FIG. 4A.
The injection resin 22 thus fed deposits on the surface of the preformed base 23 of the lid body 20 at the backside of the fixing portion 16 of the tab 15, and also the injection resin 22 fed to the area delimited by the mold 14 deposits on the surface of the preformed base 23 at that area. At the same time, the injection resin 22 fed to the area formed between the circular mold 12 and the mold 13 deposits on the surface of the preformed base 23 at that area.
Then, after the mold is cooled, the mold is opened to remove the molded product (the lid) therefrom, thus obtaining the lid 24. FIG. 8 is a plan view of the lid 24, and FIG. 9 is an enlarged cross-sectional view taken along the line B--B of FIG. 8.
As shown in FIG. 9, the injection resin 22 has not been deposited on the preformed base 23 at that portion where the mold 12 has been present and also at the area between the molds 13 and 14, and therefore these portions of the preformed base 23 are exposed to provide grooves 30 and 31.
The injection resin (layer) 22 thus deposited on the area surrounded by the circular mold 12 constitutes a tab seat 25 of the lid 24, and the through hole 18 of the fixing portion 16 of the tab 15 is filled with the injection resin 22. The fixing portion (front end portion) 16 of the tab 15 is integrally connected or fixed to the tab seat 25 by the injection resin 22 lying between the backside of the fixing portion 16 and the surface of the multi-layer base 23 disposed in opposed relation thereto.
As shown in FIG. 9, since the tab 15 is thus fixed at its front end portion 16, the other end of the tab body 17 is spaced from the preformed base 23 (although the other end is not always raised as in the drawings).
Thus, there is obtained the lid 24 which is made predominantly of synthetic resins and comprises the preformed base 23, the flap portion 27 extending from the outer peripheral edge of the bottom of the preformed base 23 to the outer end of the flange of the preformed base 23 and further projecting in parallel relation to the upstanding portion of the preformed base 23, the tab seat 25 formed or deposited on that portion of the upper bottom surface of the preformed base 23 surrounded by the flap portion 27, and the tab 15 secured at its one end to the tab seat 25.
The lid 24 has a score portion 30 of a circular shape having a substantially uniform thickness or width over the entire circumference thereof, the score portion being the exposed portion of the preformed base 23 and being formed as a result of the presence of the circular mold 12. The tab seat 25 and the flap portion 27 (the reinforcing thermoplastic resin layer 26) are spaced apart by the score portion 30.
For example, as shown in FIG. 13, the lid 24 is connected to a can-like container 42 by heat-bonding or sealing the flap portion 27 of the lid 24 to a peripheral wall 43 of the can-like container 42 by heat bonding or sealing. The peripheral wall 43 is also made of a synthetic resin. In FIG. 13, reference numeral 44 denotes a bottom lid.
The manner of opening the lid 24 will now be described with reference to FIG. 9. When the end 33 of the body 17 of the tab 15 for opening the lid (which tab is disposed inwardly of the tab seat 25) is raised or pulled upwardly as indicated by an arrow, the fixing portion 16 of the tab 15 is depressed in the direction opposite to the direction of the arrow. When the tab 15 is further pulled upwardly, the preformed base 23 is cut at the score portion 30, so that the lid 24 is opened along the outer peripheral edge 32 of the score portion 30.
FIG. 11 is a top plan view of the lid 24 after it has been opened, and FIG. 12 is a cross-sectional view taken along the line C--C of FIG. 11.
In the above manufacturing method, although the lid body 20 with the flap portion 27 connected to the end of the preformed base 23, is beforehand inserted in the injection mold 19, only the preformed base 23 may be inserted, in which case another resin flow path 29 and another gate 28 in addition to the gate 11 are formed in the injection mold 9 as indicated in phantom in FIG. 5 so that a molten resin is injected through the flow path 29 and the gate 28 either simultaneously with or separately from the injection of the molten injection resin 22 from the gate 11, to thereby form the flap portion 27.
Also, in the above manufacturing method, although the tab 15 is beforehand abutted against the inner surface of the circular mold 12 and is interposed between the molds 13 and 14 as shown in FIG. 4A, the tab 15 may be connected to the tab seat 25 at a separate step after the tab seat 25 is formed.
The molds 13 and 14 serve to hold the tab 15 in place relative to the injection mold 9 in the case where the tab 15 is beforehand mounted in the injection mold 9, and the mold 13 also serves to prevent the outer peripheral surface of the body 17 of the tab 15 from being joined or connected to the injection resin layer 22 formed between the circular mold 12 and the mold 13.
Similarly, the mold 14 disposed inwardly of the mold 13 serves to prevent the injection resin 22, fed to the area delimited by the mold 14, from being joined to the inner peripheral surface of the body 17 of the tab 15.
Therefore, in the case where the tab 15 is not mounted on the injection mold beforehand, the molds 13 and 14 are not needed, and in such a case the tab seat 25 is formed over the entire area of the upper bottom surface of the preformed base 23 surrounded by the inner peripheral end edge of the flap portion 27.
As described above, Applicant of the present application has earlier proposed the above method of manufacturing a lid for a can-like container by injection molding. In this method, the circular mold 12 is required for forming the score portion 30, and therefore even if it is desired that the width (S) of the score portion 30 be as small as possible, that is, as close to zero (S=0) as possible, this is impossible because of the structure of the mold. As a result, the surface of the preformed base 23 is exposed to the exterior in an amount corresponding to the width of the score portion 30.
The present inventors have earlier studied the relationship of the width of the score portion 30, the degree of easiness to open the lid (degree of easy opening) and a rupture strength (product drop strength) of the can after it is filled with its contents, and have found the following facts (as disclosed in Japanese Patent Application No. 146943/84).
More specifically, it is generally considered that the narrower the width of the score portion 30 is, the greater resistance to withstand a breakage is obtained. However, the reverse is the case, and the greater the width of the score portion 30, the more energy is absorbed when the can is dropped, thus obtaining a greater rupture strength. In contrast, with respect to the degree of easy opening, it is considered that the wider score portion 30 provides for a higher degree of easy opening. However, the reverse is also the case, and if the score portion 30 has a great width, the preformed base 23 is susceptible to stretch or expand, which greatly affects adversely the degree of easy opening.
Therefore, the above-mentioned synthetic resin lid, which is made predominantly of synthetic resins and can be opened without the use of any tool has such contradictory characteristics, that is, the degree of easy opening and the rupture strength. If it is intended to improve one characteristic, then the other characteristic often tend to become unsatisfactory. Thus, such difficult problems have conventionally been involved in providing a container having such a synthetic resin lid, and it has been desired that the lid should have the two contradictory characteristics of a satisfactory level.
The score portion 30 is the exposed portion of the preformed base 23 which comprises a laminate composed of an aluminum foil 39 and thin layers 40 and 41 of a synthetic resin formed respectively the opposite sides of the aluminum foil 39, as later described. Therefore, there is a risk that a hole may be formed through the score portion 30, and a foreign matter is introduced or injected into the container.
In order to reduce the width of the score portion 30 to zero, it is considered that the flap portion 27 and the tab portion 25 are formed of such different synthetic resins that these two synthetic reins will not adhere to each other, thereby enabling them to be released from each other at the boundary therebetween. However, the synthetic resin layers 40 and 41, which are made of the synthetic resin which can be heat-bonded to the tab seat 25, are used to define the outer surfaces of the preformed base 23. Therefore, it is actually difficult to select suitable synthetic resins which can be released from each other at the boundary to enable the opening of the can.
Further, in the above manufacturing method, the tab seat 25 made from the molten injection resin 22 is integrally or firmly joined to the surface of the preformed base 23, and the preformed base 23 is not sufficiently stretchable at the portion thereof underlying the tab seat 25. Therefore, with respect to the internal pressure within the can, the preformed base 23 may fail to provide a sufficient buffer effect for absorbing energy produced when the can is dropped. The result is that the impact concentrates on the score portion 30 when the can is dropped.
Further, in the above manufacturing method, as shown in FIGS. 5 and 8, in order to fully open the lid, the injection gate 11 for forming the tab seat 25 and the injection gate 29 for forming the flap portion 27 must be isolated from each other. In addition, in the case where the injection gate 29 for forming the flap portion 27 is provided, the strength of the injection mold 9 is reduced to shorten a service life of the mold 9. Moreover, in such a structure, it is difficult for the gas to escape, and during the molding operation, the score portion 30 are liable to be broken or cut.